1. The Field of the Invention
This invention relates to cleaning a semipermeable membrane used in a reverse osmosis or ultrafiltration system.
2. The Prior Art
A layer of slime, mud, salts, rust, insoluble particles or precipitates, or other foreign material often tends to deposit, after a period of use, on the surface of semipermeable membranes employed in membrane separation process systems such as reverse osmosis and ultrafiltration. Such deposits seriously reduce membrane efficiency; they block or blind the membrane surface, and reduce flux and often the salt rejection. If the membrane surfaces can not be cleaned at intervals, premature membrane replacement may be required to maintain plant output and product quality.
Many methods have been suggested or used for cleaning semipermeable membranes used in membrane process systems. These methods have included washing or flushing the membrane surfaces with a variety of solvents or solutions, often detergent solutions. Water sprays or pressurized water have been proposed to hydraulically loosen foreign material. Scrubbing as by use of sponges, soft plastic balls or other semi-rigid bodies has been suggested, often in conjunction with water or solution application.
A problem with such methods is that detergent and solvent solutions must be mild in order not to damage by chemical attack the membrane itself or the glue used in cartridge assemblies. Many detergent solutions are alkaline, which promotes hydrolysis of membrane materials such as cellulose acetate, with resulting early failure. Hydraulic or mechanical action must be carefully employed since the membrane is structurally relatively weak, and the active surface layer is characteristically extremely thin. On the other hand, many slime and mud formations may be considered to be refractory. They are often bound tightly into adhesive films, which are difficult to remove employing relatively mild solutions or dislodging methods which could otherwise be employed. Another disadvantage of the use of solvents or detergent solutions is that these may have to be carefully removed from the membrane surface by rinsing after use, to avoid possible contamination of product water, or the concentrate, which might be the wanted product as in an ultrafiltration process.
This invention employs a pure liquid for membrane cleaning, which contains no detergents or alkalis, which does not promote hydrolysis of membrane materials, which very effectively cleans mud, slime and other foreign materials from the surface of a semipermeable membrane, and which requires no rinsing after use and before restarting operation of the system.